Seat belt systems exist in a wide variety of forms, the most common being that of a continuous belt retractor system. In this system, the belt is wound on a retracting shaft inserted in a housing, is unwound to lie across a user's lap, and has a male buckle member inserted in a female buckle member. A spring bias is provided in a retractor base so as to exert a back pressure on the belt to take up any slack and hold the user firmly on the seat. A toothed retractor gear (often referred to as a ratchet wheel) in combination with a pawl is provided with the retractor shaft to prevent further unrolling of the belt once the slack has been taken up.
Although the seat belt systems are effective in restricting passenger movement in a collision, often problems are encountered when the vehicle is operated over rough terrain. When these safety belt systems are used in off-road vehicles such as tractors and earthmoving equipment, a condition known as "cinch-up" can occur. As the vehicle is driven over rough terrain, the driver will be forced down into his seat thereby causing slack in the belt. This excess slack will be taken up by the retractor mechanism and the retractor ratchet wheel and pawl will prevent further extension of the belt thereby causing the belt to tighten about the driver. Such tightening may cause discomfort and impair the driving ability of the operator.
It is desirable to provide a retractor lock mechanism for use with the safety belt system. The incorporation of such a device in a safety belt system prevents "cinch-up" by locking the retractor in position, thereby preventing take-up of the belt.
The only safety belt mechanism known to applicants which includes means for preventing "cinch-up" is that of Boland U.S. Pat. No. 3,319,998.